AC interconnection apparatus for supplying AC power from a commercial power system and from a solar cell

ABSTRACT

An AC interconnection apparatus has an input terminal for a commercial power system, a connection terminal to a solar cell, an output terminal connected to a load, an inverter for converting the output voltage of the solar cell into an AC voltage, and a current detector arranged on the load side of an AC connection point for connecting the AC output from the inverter and the commercial power. When a power value calculated on the basis of a current detection value by the current detector and the voltage of the commercial power system is less than the output power of the inverter, the output of the inverter is suppressed, thereby suppressing reverse power flow to the commercial power system. When the current value detected by the current detector exceeds a predetermined value, power supply to the load is stopped using a breaker.

This is a divisional application of application Ser. No. 09/984,391,filed Oct. 30, 2001 now U.S. Pat. No. 6,605,881.

FIELD OF THE INVENTION

The present invention relates to an AC interconnection solar powergeneration apparatus and control method therefor.

BACKGROUND OF THE INVENTION

An example of the use form of a solar power generation system is a DCinterconnection system disclosed in Japanese Patent Laid-Open No.5-108176. In this system, the output from an AC/DC converter 100 thatreceives commercial power 2 and the output from a solar cell 3 areconnected, and the DC power thus obtained is converted into AC power byan inverter 110 and supplied to a load 4, as shown in FIG. 9. Such asystem form can be regarded as a simple “load” from the viewpoint ofpreventing any reverse power flow to the commercial power system. Forthis reason, when deregulation about electric power greatly progressesin the future, individuals can freely use such a solar power generationsystem.

In the prior art, however, the capacity of the load 4 is limited by thecapacity of the inverter 110. In addition, no measures are taken againstunnecessary current leakage of the solar cell 3 due to the straycapacitance to ground, i.e., no device for safety use of the solar cell3 is disclosed.

Another example that may make it possible to freely use a system as a“load” is an AC interconnection system which prevents any reverse powerflow. Unlike the above-described system, commercial power and AC powerfrom a power generator are connected without any conversion and suppliedto a load, and the power generator is stopped upon detecting reversepower flow from the power generator to the commercial power system,using a protective relay for detecting reverse power to the commercialpower. In this prior art, the limitation on the load capacity due to theoutput inverter 110 is not present, unlike the arrangement shown in FIG.9, though the commercial power and power generator parallelly supplypower to the load. For this reason, along with an increase in powergenerated by the power generator, a usable load capacity may exceed therated load capacity to cause a load current beyond the rated value toflow, resulting in danger.

SUMMARY OF THE INVENTION

The present invention has been made to solve these problems, and has asits object to provide an AC interconnection apparatus capable of easily,safely, and efficiently using a solar cell and a control methodtherefor.

It is another object of the present invention to provide an ACinterconnection apparatus capable of preventing any reverse power flowto a commercial power system even when power generated by a solar cellincreases in an apparatus which supplies both commercial power and ACpower from the solar cell to a load, and a control method therefor.

It is still another object of the present invention to provide an ACinterconnection apparatus capable of preventing any overpower to a loadin an apparatus which supplies both commercial power and AC power from asolar cell to the load, and a control method therefor.

In order to achieve the above described objects, an AC interconnectionapparatus of the present invention comprising the structures as follows.

An AC interconnection apparatus for supplying to a load both AC powerfrom a commercial power system and an output from a solar cell,comprising: an inverter for converting the output from the solar cellinto AC power; a current detector arranged between the load and an ACconnection point for connecting the AC input from the commercial powersystem and the AC output from the inverter to detect a current flowingto the load; and control means for obtaining load power applied to theload on the basis of a voltage value at the AC connection point and thecurrent value detected by the current detector and controlling theoutput of the inverter on the basis of the load power and the outputpower of said inverter.

In order to achieve the above described objects, a control method for anAC interconnection apparatus of the present invention comprising thesteps as follows.

A control method for an AC interconnection apparatus for supplying to aload both AC power from a commercial power system and an output from asolar cell, comprising: a current detection step of detecting a currentflowing to the load between the load and an AC connection point forconnecting the AC input from the commercial power system and an ACoutput from an inverter for converting the output from the solar cellinto AC power; and a control step of obtaining load power applied to theload on the basis of a voltage value at the AC connection point and thecurrent value detected in the current detection step and controlling theoutput of the inverter on the basis of the load power and the outputpower of the inverter.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a block diagram showing the arrangement of an ACinterconnection apparatus according to the first embodiment of thepresent invention;

FIG. 2 is a view showing the outer appearance of the AC interconnectionapparatus according to the embodiment of the present invention;

FIG. 3 is a flow chart for explaining reverse power flow preventingoperation by a controller of the AC interconnection apparatus accordingto the first embodiment;

FIG. 4 is a flow chart for explaining load current breaking operation bythe controller of the AC interconnection apparatus according to thefirst embodiment;

FIG. 5 is a block diagram showing the arrangement of an ACinterconnection apparatus according to the second embodiment of thepresent invention;

FIG. 6 is a flow chart for explaining reverse power flow preventingoperation by a controller of the AC interconnection apparatus accordingto the second embodiment;

FIG. 7 is a flow chart for explaining load current breaking operation bythe controller of the AC interconnection apparatus according to thesecond embodiment;

FIG. 8 is a block diagram showing the arrangement of an ACinterconnection apparatus according to the third embodiment of thepresent invention; and

FIG. 9 is a view for explaining a prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings.

<First Embodiment>

[Outer Appearance and Connection Terminal]

FIG. 1 shows the arrangement of an AC interconnection apparatus 1according to the first embodiment, and FIG. 2 shows the outer appearanceof the apparatus.

As shown in FIG. 2, the AC interconnection apparatus 1 according to thisembodiment has a shape like a so-called table tap and has a plug-shapedconnection terminal 11 to a commercial power system, awall-socket-shaped (receptacle-shaped) load connection terminal 13, anda solar cell connection terminal 12 having a shape different from theterminal 11. The solar cell connection terminal 12 has a shape differentfrom the terminal 11 to prevent any connection error, i.e., for safety.The terminals can have various shapes as far as they prevent anyconnection error. It is preferable that the terminal 12 has a differentshape from the terminal 13 in the same point as the above describedreason. Instead of using these terminals, a screwed terminal may beused. For practical use, however, connectors are preferably used, as inthis embodiment.

[System Interconnection Inverter]

As a system-interconnection inverter 14, a well-known sine-wave PWMinverter (incorporating a commercial insulating transformer) formed froma boosting chopper and full-bridge circuit is used. As an insulatingmethod, a method called high-frequency insulation using a high-frequencytransformer may be used. In the high-frequency insulation, thetransformer can be made compact, and it is very preferable for thisembodiment. Many methods are known as a method of controlling theinverter 14, and a detailed description thereof will be omitted.Generally, many system interconnection inverters for solar powergeneration employ a control method called “instantaneous currentcontrol”. Briefly speaking, in this control method, an “output currentreference waveform” is input, and a switch circuit is controlled suchthat the reference waveform matches a current waveform to be actuallyoutput. Output power is determined by the amplitude and the phasedifference from the system voltage. Since the inverter is often used ata power factor of 1 by fixing the phase difference to 0, it can beregarded that the output is proportional to the amplitude. The inverter14 also has a maximum power tracking function and a control function ofmaintaining the input voltage from a solar cell 3, when the output fromthe solar cell 3 is maximized.

A controller 15 of the inverter 14 normally incorporates a function ofdetecting the output current and output power of the inverter 14. Inthis embodiment, reverse power flow preventing operation is performedusing this function. The capacity of the inverter 14 is 50 W, though itis appropriately selected in accordance with the capacity of the solarcell 3 connected. If the inverter 14 has a large capacity, the capacityof the usable solar cell 3 can also be increased. However, a necessaryand minimum capacity is preferable because a large capacity inevitablycauses an increase in size and cost of the apparatus.

[Solar Cell]

As the solar cell 3, a crystal-based solar cell (output power: 50 W,output voltage: 16 V) is used. When such a low-voltage module is used,the safety for human body and the like can be increased. Especially,since a low-voltage-output solar cell with an output voltage lower than30 V is excluded from electric equipment to which the ElectricityUtilities Industry Law applies, the breakdown voltage can also be madelow, and as a result, cost reduction can be expected. Additionally, itis generally supposed that such a low-voltage solar cell rarely causesan electrical shock as compared to a high-voltage solar cell and istherefore easy to handle for end users who have no special knowledge.

[Load and Commercial Power System]

As an experimental load 4, an electric bulb (60 W) is used. As acommercial power system, a 100-V/60-Hz system is used. They are notparticularly limited to the above load and system and can beappropriately selected as needed. In this embodiment, the load 4 candirectly receive power from the commercial power system 2, unlike the DCinterconnection apparatus shown in FIG. 9. Hence, the capacity of theload 4 can be set to the full rated current determined by the receptacleshapes shown in FIG. 2 or the thickness of an electrical wire. Forexample, when the shapes of this embodiment are selected, the ratedcapacity is 15 A to 20 A, so a load of 2 kVA can be connected. To thecontrary, in the above-described conventional DC interconnectionapparatus, an inverter having a capacity of 2 kVA is required to drive aload of 2 kVA, resulting in a bulky apparatus.

[Current Detector and Breaker]

As a current detector 16, a normal current transformer that is oftenused is used. The set position is between an AC connection point 17 andthe load connection terminal 13, as shown in FIG. 1. Unless the currentdetector 16 is set at this position, it cannot directly detect the loadcurrent. To detect the current, many known devices such as a deviceusing a Hall element can be selected. As a breaker 18, a commerciallyavailable breaker or power relay can be used. A compact power relay fora power circuit is used here.

[Controller]

The controller 15 controls the inverter 14 or other switch circuits, anda well-known one-chip microcomputer or sequencer can be used. In thisembodiment, as the controller 15, a commercially available one-chipmicrocomputer is used. However, any change can be made if the object ofthe present invention can be achieved. In this embodiment, the powersupply voltage of the controller 15 is obtained from a point Vs. How toobtain the power supply voltage is not particularly limited. The voltageat the connection point Vs is also used to detect the voltage of thecommercial power system, which is necessary for power calculation.

[Operation]

{circle around (1)} Reverse Power Flow Preventing Operation (ReversePower-flow Protection)

The flow chart of FIG. 3 shows an example of reverse power flowpreventing operation of this embodiment. FIG. 3 is a flow chart forexplaining reverse power flow preventing operation by the controller ofthe AC interconnection apparatus 1 according to the first embodiment.

First, in step S1, the voltage of the commercial power system, which isobtained from the connection point Vs, is detected. In step S2, the loadcurrent is detected by the current detector 16. In step S3, load powerPL is calculated from these voltage and current values. The flowadvances to step S4 to obtain output power Pi of the inverter 14 fromthe inverter 14. In step S5, input power from the commercial powersystem is calculated as the difference between the load power PL and theoutput power Pi of the inverter 14. In step S6, it is checked whetherthe input power (PL-Pi) obtained in step S5 is less than a predeterminedvalue (e.g., 0). If YES in step S6, the flow advances to step S7 todecrease the output current of the inverter 14 by a predeterminedamount. This operation is repeated until the power input from thecommercial power system becomes equal to or more than the predeterminedvalue in step S6.

In the first embodiment, the current detector 16 is arranged between theAC connection point 17 and the load connection terminal 13. Hence, toprevent any reverse power flow, the process of calculating the inputcommercial power from the output of the inverter 14 and the power of theload 4 is necessary. However, with this process, the object “reversepower flow prevention” can be achieved. In addition, the single currentdetector 16 suffices, resulting in simple arrangement of the ACinterconnection apparatus 1.

{circle around (2)} Load Over-current Breaking Operation (Over LoadProtection)

As described above, in the first embodiment, since the current flowingto the load 4 is directly measured, it is easy to detect a loadover-current and break the current when the load over-current has apredetermined value or more. The operation flow is shown in FIG. 4.

FIG. 4 is a flow chart for explaining load current control by thecontroller of the AC interconnection apparatus 1 according to the firstembodiment.

First, in step S11, the load current flowing to the load 4 is detectedby the current detector 16. In step S12, it is determined whether theload current exceeds a predetermined value. If YES in step S12, the flowadvances to step S13 to open the breaker 18 to execute breakingoperation.

It is important here that since the current detector 16 is arrangedbetween the AC connection point 17 and the load connection terminal 13,the load current can be controlled by such considerably simple controloperation.

{circle around (3)} Checking Resistance about Stray Capacitance toGround

In the arrangement shown in FIG. 1, the solar cell 3 is insulated fromthe commercial power system. Hence, any unnecessary operation of theelectrical leakage breaker can be prevented. This will be brieflydescribed below.

As is known, in a solar power generation system in which the power inputterminal from the solar cell is uninsulated from the commercial powersystem, and the stray capacitance to ground is large, generally, anunnecessary leakage current flows. As an experiment, in the ACinterconnection apparatus 1 having the arrangement shown in FIG. 1, a100-μF capacitor was connected between the solar cell 3 and ground. Whenthe presence/absence of a leakage current was checked, no leakagecurrent was observed. From this viewpoint, in the apparatus of thisembodiment, since no leakage current flows, the electrical leakagebreaker does not perform any unnecessary operation, and the safety isvery high. Since the stray capacitance changes depending on the shape orinstallation form of the solar cell 3 or the weather, the ACinterconnection apparatus 1 of this embodiment, which can be connectedto the solar cell 3 and used without taking the presence of suchundefined stray capacitance into consideration, is very convenient forpeople who have no special knowledge.

<Second Embodiment>

The second embodiment of the present invention will be described below.

An AC interconnection apparatus 1 a according to the second embodimentof the present invention has the arrangement shown in FIG. 5. The secondembodiment is different from the first embodiment in the position of acurrent detector 16. In the second embodiment; the current detector 16is arranged between an AC connection point 17 and an input terminal 11of a commercial power system 2. When the current detector 16 is arrangedat this position, the input/output current at the terminal 11 can bedirectly measured. The components shown in FIG. 5 are the same as in theabove-described first embodiment (FIG. 1), and a detailed descriptionthereof will be omitted. Operation different from the first embodimentwill be described below.

[Operation]

{circle around (1)} Reverse Power Flow Preventing Operation (ReversePower-flow Protection)

FIG. 6 is a flow chart showing reverse power flow preventing operationby a controller 15 of the AC interconnection apparatus 1 a according tothe second embodiment of the present invention.

First, in step S21, the current value of the commercial power system isdetected by the current detector 16. In step S22, the voltage value ofthe commercial power system at a connection point Vs is obtained. Instep S23, input power PL is calculated from the current value detectedin step S21 and the system voltage Vs. In step S24, it is checkedwhether the power value PL is negative or is less than a predeterminedvalue in consideration of a slight measurement error and the like. IfYES in step S24, the flow advances to step S25 to decrease the outputcurrent and amplitude instruction value of an inverter 14, therebysuppressing the output of the inverter 14. The amplitude instructionvalue is gradually decreased by a predetermined amount until no reversepower flow is observed. The output of the inverter 14 may be stopped asneeded. When the current detector 16 is arranged at the position shownin FIG. 5 of the second embodiment, the reverse power flow preventingoperation can be more simplified.

{circle around (2)} Load Over-current Breaking Operation (OverloadProtection)

FIG. 7 is a flow chart showing processing of controlling a load currentby the controller 15 of the AC interconnection apparatus 1 a accordingto the second embodiment of the present invention by detecting a loadover-current.

First, in step S31, the input current from the commercial power systemis measured by the current detector 16. In step S32, the output currentof the inverter 14 is detected. In step S33, a load current flowing to aload 4 is obtained from the sum of the input current obtained in stepS31 and the output current of the inverter 14 obtained in step S32. Instep S34, it is checked whether the load current has a predeterminedvalue or more. If YES in step S34, the flow advances to step S35 to opena breaker 18 to break the current flowing to the load 4.

To prevent any reverse power flow when the current detector 16 is set atthe position of the second embodiment, the process of calculating theload current from the input current from the commercial power system andthe output current of the inverter 14 is necessary. However, with thisprocess, the object “load over-current breaking” can be achieved, andthe safety increases. In addition, the single current detector 16suffices, resulting in simple arrangement of the AC interconnectionapparatus.

<Third Embodiment>

The third embodiment of the present invention will be described below.

FIG. 8 is a block diagram showing the arrangement of an ACinterconnection apparatus 1 b according to the third embodiment of thepresent invention. The same reference numerals as in the above-describedembodiments denote the same parts in FIG. 8, and a description thereofwill be omitted.

In the third embodiment, as shown in FIG. 8, two current detectors (16Aand 16B in FIG. 8) are used. With this arrangement, the load current andthe input/output current of the commercial power system can be directlymeasured. Hence, load current breaking operation by the loadover-current can be executed as in the above-described flow chart ofFIG. 4. Reverse power flow preventing operation can be executedaccording to the flow chart shown in FIG. 6. An inverter 14 need nothave a function of detecting the output current or output power, and thestructure of the inverter 14 can be simplified.

Although the embodiments have been independently described above, thepresent invention is not limited to this, and an appropriate combinationof the arrangements of the embodiments is also incorporated in thepresent invention.

The present invention is not limited to the above embodiments andvarious changes and modifications can be made within the spirit andscope of the present invention. Therefore, to apprise the public of thescope of the present invention, the following claims are made.

What is claimed is:
 1. An AC interconnection apparatus for supplying ACpower to a load from a commercial power system and/or an output from asolar cell, comprising: a plug-shaped connection terminal for thecommercial power system; a receptacle-shaped load connection terminal;and a solar cell connection terminal having a shape different from saidplug-shaped connection terminal for the commercial power system.
 2. Anapparatus according to claim 1, wherein said solar cell connectionterminal has a shape different from said receptacle-shaped loadconnection terminal.